Method and arrangement for performing measurements of the topography of a surface by means of a thermal emission from the surface

ABSTRACT

Method and arrangement for performing measurements of the topography of a surface ( 20 ), such as topography of an eye surface ( 20 ), wherein projecting means ( 1, 12 ) for projecting an image onto said surface ( 20 ) comprises a projection light source ( 1 ), and wherein at least a fraction of light leaving the surface ( 20 ) as a result of said projection is received using one or more receiving units ( 31, 32 ), such as charged coupled device (CCD) based cameras. The topography of the surface ( 20 ) is determined by analysis of said fraction of light leaving the surface ( 20 ), due to thermal emission and the image projected onto the surface ( 20 ) is projected with light comprising a colour for which the surface ( 20 ) is opaque, such as infrared light.

FIELD OF THE INVENTION

The present invention relates to a method for performing measurements ofa topography of a surface, such as the topography of an eye surface,wherein an image is projected onto said surface from at least oneprojection light source using projection means, wherein at least afraction of light leaving the surface as a result of said projection isreceived using one or more receiving units, such as charged coupleddevice (CCD) based cameras, wherein measurement of said topographyrelates to surface mapping of said surface, wherein said topography ofthe surface is determined by analysis of said fraction of light leavingthe surface, and wherein said fraction of light leaving the surface iscomprised of light radiated by the surface due to thermal emission.

The invention further relates to an arrangement for performingmeasurements of the topography of a surface, such as topography of aneye surface, wherein measurement of said topography relates to surfacemapping of said surface, said arrangement comprising projection means,which projection means comprise at least one projection light source forprojecting an image onto the surface, further comprising one or morereceiving units for receiving at least a fraction of light leaving thesurface as a result of said projection, such as charged coupled device(CCD) based cameras, and means for analysis of said fraction of lightleaving the surface for determining the topography of the surface.

BACKGROUND OF THE INVENTION

A method and arrangement of this type is for example disclosed in U.S.Pat. No. 6,024,449, wherein a grid pattern of pulsed monochromaticpolarized light is projected onto a semi-difuse target surface, such asthe de-epithelialized cornea of an eye undergoing photo refractivekeratometry (PRK) or photo therapeutic keratometry (PTK), for performinghigh speed topography measurements on said target. In addition, themethod and arrangement disclosed include respectively enable measurementof regions of overtemperature (hot spots) on the target surface using aquantum well infrared photodetector.

Many industrial, scientific and medical processes involve themeasurement of the topography of surfaces for variety of applications.In most cases the accuracy of the measurements is of great importancefor the quality of the output of the process mentioned. A specific typeof surface topography measurements involves the measurements of curvedsurfaces, as is applied, for example, in ophthalmology where thetopography of the corneal surface of the eye provides an indication ofthe quality of the eye and possible deflections from a healthy eye.

Some methods of measuring the topography of surfaces are based onspecular reflection, e.g. a wave-front of a collimated coherent beam oflight is projected on the surface and its reflected part is comparedwith an undisturbed reference beam (interferometric measurement) or apattern is projected or mirrored by the surface and its reflection iscompared with a reference pattern.

The limitations of these methods are that the accuracy is greatlyreduced when both convex and concave curvatures are present on thesurface or when rays are reflected outside the aperture of the sensor.

A solution is the application of moiré methods. Moiré methods are aversatile set of techniques for in-plane and out-of-plane deformationmeasurements, topographic contouring, and slope and curvaturemeasurement. The basis of moiré methods are grids, which for use in eyesurface topography may enable the projection of a pattern of lines ontothe surface. Detection of the line pattern on the surface and overlayingthe line pattern by an undisturbed reference pattern visualises themoiré pattern, which comprises the required information about thetopography of the surface.

For detection of the projected pattern, a diffusely radiating surface isdesired for this technique. For specular reflecting surfaces like theeye, in prior art, these surfaces are transformed in a diffuselyradiating surface through the application of fluorescein on the surface.A diffuse reflection of the projected pattern can then be detected bythe detection means.

A disadvantage of this technique is that it is marginally invasive.Especially for ophthalmologic purposes this is not desired since theapplication of fluorescein onto the eye disturbs the tear film and makesit difficult to see references, such as the pupil or the iris, on orbeneath the surface of the eye. Another drawback of this technique isthat it does not work on dry surfaces, such as dry eyes.

A good example of a method and arrangement used for eye topography,wherein a derivative of moiré methods and the application of fluoresceinis used, is described in European patent EP 0 551 955. This documentdescribes some features of such a kind of imaging in surface topographymeasurements, especially in relation to eye and corneal surfacetopography, in more detail.

International patent application no. WO 02/45578 is directed to a methodfor determining the topography of a surface of a biological tissue,comprising the projection of an image onto this surface. The projectionis performed with either ultra-violet (UV) or infra-red (IR) light. Themethod comprises measurement of scattered radiation from the surface asa result of said projection. The method also proposes the use of a thinfluorescent layer for increasing the yield of light to be measured. IfUV light is used for determining the topography of the surface, themethod may be based on autofluorescence of the surface.

As mentioned above, the use of a fluorescent layer may not always bedesired, especially not in application directed to determining thetopography of a corneal surface as the fluorescein disturbs the tearfilm on said surface. Disadvantages of the use of UV light, especiallyin the range for which autofluorescence occurs, is that at thesefrequencies a surface such as an eye surface may be damaged due toprotein denaturation, which may cause actinic conjunctivitis. If neithera fluorescent film nor UV light is used, but instead the method is onlybased on analysis of scattered radiation from the surface alone using IRlight, the yield of light to be analysed may be very small, since theparticles that are to scatter the radiation back, are for a large partsmaller than the wavelength used. A person skilled in the art mayappreciate that such a scattering process will be inefficient.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andarrangement for performing measurements of the topography of a; surface,such as an eye or corneal surface, which alleviates the problemsdescribed above and wherein the surface remains undisturbed, thusreducing the chance of disturbing measuring results by the measuringmethod itself.

These and other objects and advantages achieved by the present inventionin that there is provided a method as described in claim 1.

Projecting the image using, for example, infra-red light providesseveral benefits. For this colour the eye is not only fully opaque, butthe photons that are absorbed by, for instance, a tissue are convertedinto heat. This heat causes thermal excitation of the matter in thetissue generating radiation comprising wavelengths other than theoriginal wavelength that was projected onto the surface. The light thatis not absorbed by the tissue, but may instead be reflected back to thereceiving unit (specular reflection, scattering), is still comprised ofits original wavelength. This natural process enables filtering of thereflected light from the signal by for example an optical filter ordichroic mirror, leaving only the fraction of light caused by thermalemission to be detected by the detection means. It will be appreciatedthat this latter fraction comes from the diffusely radiating surface,and provide the diffuse image that can be used in surface mappingmethods. This method may in particular be efficient if mid-IR light isused, since the thermally emitted fraction is relatively large for thesefrequencies. Note further that invention is not limited to the use ofmid-IR since other wavelengths may be used for different surfaces thanthe corneal surface taken here as an example.

According to an embodiment of the present invention, the image projectedonto the surface is projected with light comprising a colour for whichthe surface is opaque.

The advantage of using light of such a colour is that this illuminationcan be applied to eyes with a normal tear film as well as dry eyes anddry surfaces, whereas methods of prior art, such as methods using theapplication of fluorescent substances on the corneal surface, cannot beused in dry eyes. In addition, the eye will not register the light usedto perform the measurements and hence it will be disturbed by themeasurement. Both the eye, as well as the results of the measurement areundisturbed by the way of measuring. Also, due to the transmission andscattering properties of the different tissues present in the eye, underthe right conditions only the shape of the outer corneal epithelium andconjunctiva are mapped.

In an embodiment of the invention an ambient light source is used toenable detection of references on the surface using one or morereceiving units. The ambient light source may radiate light of a colourfor which the surface is at least partly transparent, for instance inorder to reveal structures and references that are present directlyunderneath the surface.

In a preferred embodiment the light radiated by the ambient light sourceis near-infrared (near-IR) light (wave length λ<1,3 μm) and the surfaceis a corneal surface of an eye.

The advantage of the above used ambient light source and in particularlythe near-IR light source that is used in corneal surface topography isthe revelation of structures underneath the surface, such as the pupilor the iris of the eye. When the pupil is visible, it can serve as adistinct landmark for alignment after the measurement. For this purposeit is necessary that contraction of the pupil can be controlled by theoperator of the measuring device. By using near-IR light in combinationwith the invention, the pupil is not influenced by the measuring lightnor the ambient light source, and references are revealed withoutintroducing disturbances. The operator may, for example, controldynamics of the measured subject, such as contraction of the pupil,using other methods, for instance the use of an additional (visible)light source in combination with the invention.

In another embodiment of the invention the fraction of light leaving thesurface is received by a plurality of receiving units. In thisembodiment, the receiving units have been accurately focused, magnifiedand aligned for the purpose of receiving a suitable image of sufficientquality at a preset distance. Said embodiment may, for example, bearranged to measure the surface topography of a surface located at apredetermined distance from the front of the measuring arrangement(since it may be a curved surface, the distance meant here is theaverage distance). The surface of the object to be measured may be fixedin space, while the arrangement may be (slightly) moveable, or viceversa.

In the above mentioned arrangement, the surface to be measured has to bebrought in position, relative to the arrangement, such that (on average)it coincides with the ‘plane of focus’ of the arrangement. Thearrangement may therefore be equipped with means for adjusting theposition relative to that of a surface to be measured.

For adjusting, the embodiment comprises means for projecting referencesonto the surface and at least some of the projection angles of theprojected references are different from each other. The references areprojected such that they form an easy recognisable pattern in the ‘planeof focus’. In addition, the angles of projection are such that in anyplane, parallel to said ‘plane of focus’ but at a small distance spacedapart thereof, the pattern formed by the references is recognisablydifferent.

By adjusting the position of the arrangement relative to the position ofthe surface, one can easily establish the distance for which the surfacecoincides (on average) with the ‘plane of focus’, by evaluating thepattern formed by said references.

The triangulation method described above can be summarized by thefollowing steps: projecting a plurality of references onto the surfacewith at least partially different projection angles such that a knownpattern is formed by the references if projected on the ‘plane offocus’, adjusting the distance between object and arrangement as toapproximate to the desired pattern as close as possible.

In a preferred embodiment near-IR light is used for projecting thereferences used for triangulation or, when the surface is an eye surfacecomprising a corneal surface to be measured, the references may beprojected onto a region outside the pupil, such as the conjunctiva.

By focussing on the conjunctiva adjacent to the corneal tissue novisible light load for the patient is present. By using near-IR lightthe chances of response of the eye to the focussing are further reduced.

In a specific embodiment the projected image of a grid is flashed ontothe surface, and the receiving units are synchronised with this flash.

The advantages of flashing the image onto the surface is that movementartefacts are frozen by the flash.

In another embodiment a series of flashes is projected onto the surfaceenabling determination of dynamics of the topography of the surface.

In an other embodiment of the invention the emission component isseparated from the excitation component of the light, and the excitationcomponent of the light is used to synchronize the receiving units with aflashed projection.

Note that specular reflection component of the light may be suppressedby the use of crossed polarizing optics, for instance by a verticaloriented polarizer placed in the projector and a horizontal orientedpolarizer near the receiving unit. Note that the thermally emittedfraction of light, used in the analysis of the topography will not beaffected by the polarisation filter.

For safety purposes it should be noted here that the use of UV lightmay, for specific use of the invention for corneal surface topographymeasurement, damage the corneal surface as it may affect proteinstructures in the corneal tissue. Although dependent on the applicationof the invention, use of the invention with UV light is not ruled out,for use on corneal surfaces it is however not advised.

It will be appreciated that the projected pattern may be created by avariety of means other than grids. A slid or a double slid or any otherinterferometric technique may be used to create an interference patternsuitable for grid projection methods. In a certain embodiment theinterference pattern used is a sinus-shaped fringe pattern.

The invention is not limited to the use in grid projection or moirémethods. The principle of using light for which the surface is opaque isadvantageous in many other techniques wherein optics are used todetermine the topography of a surface. The use of this invention forsimilar purposes may be suggested by the additional advantages of theuse of the invention for ophthalmologic purposes as described above. Theinvention is not limited to the use of IR or UV light, or similarcolours. It will be appreciated that, in relation to surface topography,the concept of adapting the wavelength of light or radiation to thecharacteristics of the measured subject is widely applicable in avariety of cases, especially for medical purposes. Examples arefluorescent angiography as used for retinal blood flow measurement ortechnical purposes such as for aspheric (contact) lenses.

The above-mentioned and other features and advantages of the inventionare illustrated in the following description of some embodiments of thepresent invention, with reference to the enclosed drawings. Systemsarranged for deploying above-mentioned method are regarded as anembodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic top view of an arrangement according to thepresent invention;

FIG. 2 shows a schematic side view of the projection unit of thearrangement of FIG. 1;

FIG. 3 is a schematic top view of the principle of the triangulationmethod of the present invention for adjusting the distance between thereceiving units and the surface to be measured.

FIG. 4 is a double logarithmic diagram of the dependency of theabsorption coefficient of sea water on the wave length of light (takenfrom “Sources of radiation—The Infrared & Electro-Optical SystemsHandbook Vol. 1’, G. J. Zissis (editor), SPIE, 1993, page 256);

FIG. 5 shows a projected pattern on a corneal surface of which thetopography is being measured;

FIG. 6 shows an image acquired with fast Fourier analysis of the patternof FIG. 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An arrangement 14 according to the present invention is shown in FIGS. 1and 2. The arrangement has three main functions: projection of a linepattern on the corneal surface 20, triangulation focussing, andvisualisation of references by using ambient light sources 18 and 19.FIG. 1 shows a schematic top view-of the arrangement, and FIG. 2 shows aside view of the projection unit generally indicated as 12.

Projection on the surface is performed by generating a line patternprojection through light source 1, aperture 2, lens 3, grid 4,telecentric lens 5, (dichroic) mirrors 7 and 8, and telecentric lens 6.The image leaving telecentric lens 6 is projected onto the cornealsurface 20. The diffuse image of the grid 4 on the corneal surface 20 ispicked up through telecentric lenses 25 and 26, filters 27 and 28,telecentric lenses 29 and 30 and cameras 31 and 32. The signal ofcameras 31 and 32 is transmitted via lines 37 and 38 to frame grabber 34and further processed by PC 35, after which it is visualised on monitor36. Aperture 2 may be a 3 mm wide slid. Grid 4 may comprise thesinus-shaped line pattern with a line density of about 3 lp/mm (linepairs per millimetre), which lines run at right angles through theprojection axis. Mirrors 7 and 8 are dichroic mirrors that reflect thelight coming from projection light source 1. Filters 27 and 28, in theoptical path between cameras 31 and 32 and eye 20, are added to blockthe light caused by specular reflection of the projected image. In thecase that projection light source 1 is a mid-IR light source, only thediffuse image caused by thermal radiation due to the absorbed IRradiation by the tissue, should be recorded by cameras 31 and 32. Thelight caused by thermal radiation typically has an other wave lengththan the light caused by specular reflection. The light caused byspecular reflection is of the same wave length as the light coming fromthe projection source, and can thus be filtered out easily by filters 27and 28.

Frame grabber 34 subsequently captures the frames coming from the firstcamera 31 and the second camera 32 and sends these images to PC 35. Theimage may be viewed on monitor 36. PC 35 may determine the gridfrequency of the projected grid on the image received and compare theresults to the original frequency of the grid used. The results willcontain desired information about the surface topography. Alternatively,PC 35 may combine the two images revealing a moiré pattern which can beviewed on monitor 36.

In order to freeze rapid eye movements and to minimize thermal diffusionduring the recording process, the illumination time can be reduced, forinstance to about 1 ms. The flash thus created can be synchronized withthe mid-IR cameras 31 and 32. In a specific embodiment two subsequentflashes can be synchronized with the first camera 31 and the secondcamera 32 subsequently, such that the first camera records an odd fieldand the second camera records an even field, which in term can beprocessed by the frame grabber 34 and the PC 35. The odd field may, forexample, be comprised of the odd picture lines of the image, and theeven field of the even picture lines. Combining both fields may thenyield an image revealing a moiré pattern.

The second functionality of arrangement 14 of FIG. 1, recording ofreference patterns underneath the corneal surface, such as pupil, irisand conjunctiva, is here performed by the addition of two ambient lightsources 18 and 19 which may, according to the invention, provide near-IRambient light for which the eye is at least partly transparent. For thispurpose, recording of these references is performed by camera 11 whichreceives reflected near-IR light coming through lens 6, and is reflectedby mirror 8 onto dichroic mirror 7. Dichroic mirror 7 transmits near-IRlight which reaches camera 11 through lens 10.

The arrangement described above may be aligned, focused and magnified inorder to project and receive suitable images at fixed distances at afixed distance from the image receiving means. The arrangement mayfurther comprise supporting means in which an object comprising asurface to be measured may be placed. The distance between the surfaceto be measured and the receiving means, after the object has been placedin front of the arrangement, must be close enough approximate to thefixed distance for which the arrangement may have been aligned, focusedand magnified as mentioned above. For this purpose the supporting meansor the arrangement as a whole my be arranged for calibrating thedistance between the surface to be measured and the receiving means(this can be done by moving the object or moving the measuring device,as will be appreciated).

In order to determine whether the distance between the receiving meansand the surface is equal to the distance for which the arrangement isable to project and receive suitable images, a triangulation method maybe used as part of the invention.

FIG. 3 schematically shows the principle of the triangulation methodused for calibrating the distance between receiving means and surface.The arrangement is aligned, focused and magnified in order to projectand receive a suitable image at the ‘plane of focus’ 44, indicated by adotted line as the ‘plane of focus’ 44 is not physically present.Reference projection means 40-43 projecting a small spot as provided bynear-IR lasers 21 and 22 in FIG. 1, project a plurality of referencepoints onto the surface to be measured. The reference projection meansare arranged such that the points projected onto a imaginary projectionplane in the ‘plane of focus’ 44 form a recognisable pattern, and inaddition, the pattern formed by these points in any other plane isrecognisably different.

The optical paths of the reference projection means 40-43 are shown asthe dotted lines 45-48. The angle of projection of these optical pathsrelative to the plane of focus is indicated by α₁ for path 45 ofprojection means 40, α₂ for path 46 of projection means 41, α₃ for path47 of projection means 42, α₄ for path 48 of projection means 43. Asurface to be measured may be a corneal surface of an eye and isindicated in FIG. 3 as 49A, B and C, wherein A, B and C indicate threepossible situations. The images received by receiving means (not shown)for the three situations are indicated by 55A, B and C. In all theseimages, 56(A, B and C) indicates the circumference of an iris and 57(A,B and C) is the pupil of the eye. The reference points are indicated by50-53 A, B and C for each situation A, B and with an additional prime(“′”) to indicate the representation of the reference on screen inimages 55A, B and C.

The optimal situation, the situation wherein the surface 49Bapproximately coincides with the ‘plane of focus’ 44, at least closeenough to acquire the desired accuracy in the results of themeasurement, is here indicated by situation B. The optical paths 45 and47 cross (but do not intersect) each other at different heights, as wellas optical paths 48 and 46. Points 50B-53B recognisably form the cornersof a rectangle, as can be seen in image 55B indicated by spots 50B′,52B′, 51B′ and 53B′.

Situation A shows the situation wherein the surface 49A is located infront of the ‘plane of focus’ 44, such that the distance betweenreceiving means (not shown) and surface 49A is too small. In situationA, the reference points 50A, 52A, 51A and 53A do not form a rectangle asin the case of situation B. In this example the projection angles α₂ andα₃ of optical paths 46 and 47 respectively relative to the ‘plane offocus’ 44 are no straight angles (of approximately 90°), but are insteadsmaller and larger respectively. Therefore, moving surface 49A more tothe front will cause reference points 51A and 52A to move closer to eachother, while moving surface 49A backwards in the direction of the ‘planeof focus’ 44 will cause reference points 51A and 52A to move away fromeach other. In situation A, the image points 50A′, 52A′, 51A′ and 53A′will form the corners of a trapezium in image 55A. An operator of thearrangement recognises this and may increase the distance betweenreceiving means (not shown) and surface 49A, until the situationindicated by B is achieved.

Situation C shows a likewise situation as A, but in C the distancebetween the surface 49C and the receiving means (not shown) is toolarge, placing surface 49C behind the ‘plane of focus’ 44. Here,reference points 52C and 51C are further away from each other than 50Cand 53C, such that in image 55C the reference points 50C′, 51C′, 52C′and 53C′ form the corners of a trapezium. Similar as in situation A, anoperator of the arrangement knows that the distance between surface 49Cand receiving means (not shown) must be decreased.

As will be appreciated, other situations may be possible wherein it maybe clear to an operator how to calibrate the distance between thereceiving means and the object. In addition, it can likewise bevisualised that a surface like 49 may be tilted, rotated or moved withinthe ‘plane of focus’ 44 (left, right, up, down or a combinationthereof). For example, when the surface 49 may need to be tilted anasymmetric pattern may be seen in image 55.

A specific embodiment visualises (when the distance or orientation iscalibrated correctly, as in situation B) the corners of a rectangledefining the surface that will be measured by the arrangement. Thisenables easy positioning of the arrangement relative to the object.

The arrangement described above, presented in FIG. 1 and FIG. 2, may besuitable for mid-IR radiation, but can easily be adapted for use withultra violet (UV) projections or other colour (such as blue light). Forinstance, using a UV light source 1, a polarizer 17 may be placed infront of lens 6 and filters 27 and 28 may be replaced by polarizationfilters, of which the direction of polarization is perpendicular to thepolarization direction of polarizer 17. The function of this set ofpolarizers is to remove light caused by specular reflection on thecorneal surface 20. Additionally, it will be appreciated that using a UVlight source 1, dichroic mirrors 7 and 8 are to be replaced by suitabledichroic mirrors.

Optionally, and especially for use in an arrangement suitable forprojecting blue light, a fixation light source 9 can be placed in astraight line behind mirror 8 on the axis between mirror 8 and thecorneal surface 20. The near-IR sources 17 and 18 as well as the near-IRprojection lasers 21 and 22 not necessarily need to be replaced in casea different colour of light is used in the main projection source 1.

FIG. 4, which is taken from “Sources of radiation—The Infrared &Electro-Optical Systems Handbook Vol. 1’, G. J. Zissis (editor), SPIE,1993, page 256, shows a diagram which depicts the dependency of theabsorption coefficient k on the wavelength of light λ for sea water. Theabsorption coefficient of sea water is comparable to the absorptioncoefficient of the corneal tissue of the eye. As can be seen in thediagram, for visible light the region generally indicated with II(approximately between 0.4 and 0.8 μm) the eye is transparent. However,in the IR-region indicated with I, for wave length larger than 0.8 μmthe absorption coefficient increases rapidly (note the doublelogarithmic scale), thus the eye is opaque for these frequencies around0.8 μm, in the near-IR region, the eye is still partly transparent. Inthe ultra violet region, region III, with wave lengths smaller thanapproximately 0.4 μm, the corneal tissue starts to become opaque forfrequencies smaller than 0.3 μm. However, for medical use and safetypurposes, UV light with a wave length longer than 0.2 μm is notpreferred, as it may permanently damage the tissue.

FIG. 5 shows an image of a projected grid pattern on an eye surface inan arrangement as an example of the present invention. The image isreceived by receiving means that are placed under a different angle(with respect to their optical path to the surface) than the projectionmeans.

FIG. 6 shows a height topography map of an eye surface calculated bycombination of the image of FIG. 5 with a similar image taken from adifferent angle (or with a different angle of projection to thesurface). The images shown in FIG. 5 and 6 have been created as examplesto explain the arrangement according to the present invention.

An arrangement according to the present invention is directed towardsthe method and arrangement disclosed in the amended claims. Thearrangement described above, comprised of a single projection unit andtwo camera's may be embodied different. A possible amendment of theembodiment described is the use of a plurality of projection units forprojecting one or more grid patterns onto the surface, or the use of aone or more receiving units (for example, one camera and two projectionunits).

It will be appreciated that numerous modifications and variations of thepresent invention are possible in the light of the above teachings. Itis therefore understood that within the scope of the attached claims,the invention may be practised otherwise than specifically describedherein.

1. Method for performing measurements of a topography of a surface, suchas the topography of an eye surface, wherein an image is projected ontosaid surface from at least one projection light source using projectionmeans, wherein at least a fraction of light leaving the surface as aresult of said projection is received using one or more receiving units,such as charged coupled device (CCD) based cameras, wherein measurementof said topography relates to surface mapping of said surface, whereinsaid topography of the surface is determined by analysis of saidfraction of light leaving the surface, and wherein said fraction oflight leaving the surface is comprised of light radiated by the surfacedue to thermal emission, characterised in that, said analysis fordetermining said topography of die surface is performed on said lightradiated by the surface due to thermal emission.
 2. Method according toclaim 1, wherein at least one of the receiving units only receives saidfraction of light leaving the surface during thermal excitation of thesurface.
 3. Method according to claim 1, wherein said fraction of thelight leaving the surface further comprises reflected light that isreflected by the surface, and wherein said reflected light is removedbefore said analysis of said fraction of light leaving the surface. 4.Method according to claim 1, wherein the surface is at least part of thesurface of a human or animal eye.
 5. Method according to claim 1,wherein the image projected onto the surface is projected with lightcomprising a colour for which the surface is opaque.
 6. Method accordingto claim 5, wherein said colour for which the surface is opaquecorresponds to a colour of infrared (IR) light.
 7. Method according toclaim 6, wherein mid-IR light is used for projecting said image on thesurface.
 8. Method according lo claim 1, wherein said projection meansflashes the image onto the surface, and wherein at least one of saidreceiving units is synchronised with said projection means.
 9. Methodaccording to claim 8, wherein said projection means projects the imageduring a series of flashes onto the surface, enabling determination ofdynamics of the topography of the surface.
 10. Method according to claim8, wherein said flashing of said image is used to synchronise the atleast one of said receiving units.
 11. Method according to claim 1,wherein illumination of the surface by an ambient light source enablesdetection of structures on or underneath the surface using said one ormore receiving units.
 12. Method according to claim 11, wherein saidambient light source radiates light of a colour for which the surface isat least partly transparent.
 13. Method according to claim 11, whereinthe surface is at least part of an eye surface, and wherein the lightradiated by said ambient light source is near-IR light.
 14. Methodaccording to claim 1, wherein a plurality of receiving units are usedfor receiving said fraction of light leaving the surface, wherein saidreceiving units are arranged for receiving a desired image of saidfraction of light at a fixed distance from said surface, and whereinplacing the surface at said fixed distance for receiving the desiredimage at least comprises the steps of: projecting a plurality ofreferences onto the surface along an optical path using referenceprojection means, which references are projected such that at least oneof the optical paths of said reference projection means is at an anglewith at least one other of said optical paths of the referenceprojection means, and such that if the references are projected on thesurface at said fixed distance to the receiving units, a recognisablepattern is formed on die surface by said references, adjusting thedistance between surface and receiving units such that said referencesform said recognisable pattern on the surface.
 15. Method according toclaim 14, wherein near-IR light is used for projecting said referencesonto the surface.
 16. Method according to claim 14, wherein the surfaceis an eye surface comprising a corneal surface, and wherein pupil; irisand conjunctiva are comprised underneath said surface, and wherein saidmore than one reference is projected onto a region of the conjunctivaoutside a region of the corneal surface.
 17. Method according to claim1, wherein said image projected onto the surface is an interferencepattern provided by any of a group of a grid, a slit, a double slit, aninterferometer, and other means for creating an interference pattern.18. Method according to claim 17, wherein said interference pattern usedis a sinusoidal fringe pattern.
 19. Method according to claim 1, whereina Moire method, Fourier analysis methods or other profilometric methodsare used for determining the topography of the surface.
 20. Arrangementfor performing measurements of the topography of a surface, such astopography of an eye surface, wherein measurement of said topographyrelates to surface mapping of said surface, said arrangement comprisingprojection means, which projection means comprise M least one projectionlight source for projecting an image onto the surface, furthercomprising one or more receiving units for receiving at least a fractionof light leaving the surface as a result of said projection, such ascharged coupled device (CCD) based cameras, and means for analysis ofsaid fraction of light leaving the surface for determining thetopography of the surface, characterised in that, said analysis meansfor determining the topography of the surface are arranged for analysinglight radiated by the surface due to thermal emission.
 21. Arrangementaccording to claim 20, further comprising filtering means for filteringsaid fraction of light leaving said surface, said filtering means beingarranged for transmission of light that is radiated by the surface dueto thermal excitation.
 22. Arrangement according to claim 20, whereinsaid projection means are arranged for flashing said image onto saidsurface.
 23. Arrangement according to claim 22, comprising means forlimiting a period of time for which at least one of said receiving unitsreceives said fraction of light leaving the surface such that saidperiod of time is approximately the duration of thermal emission as aresult of said flashed image on said surface.
 24. Arrangement accordingto claim 20, wherein said projection light source emits light of acolour for which the surface is opaque.
 25. Arrangement according toclaim 24, wherein said colour for which the surface is opaquecorresponds to a colour of infrared (IR) light.
 26. Arrangementaccording to claim 25, wherein mid-IR light is used for projecting saidimage on the surface.
 27. Arrangement according to claim 20, comprisingmeans for synchronising said receiving units with said projection means.28. Arrangement according to claim 20, further comprising an ambientlight source and means for detecting references on said surface. 29.Arrangement according to claim 28, wherein said ambient light sourcecomprises a near-IR light source.
 30. Arrangement according to claim 20,comprising a plurality of receiving units, and further comprising meansfor projecting more than one reference onto the surface, and means forconstructing a composite image from images received by said receivingunits.
 31. Arrangement according to claim 20, wherein said projectingmeans comprises means for projecting an interference pattern onto saidsurface.